1. Field of the Invention
The present invention relates to a color cathode-ray tube and, more particularly, to a color cathode-ray tube in which a reinforcing bead is formed on the effective area of a shadow mask to improve the mechanical strength of the shadow mask.
2. Description of the Related Art
Generally, a color cathode-ray tube has an envelope constituted by a substantially rectangular spherical panel and a funnel integrally bonded to this panel. A phosphor screen comprising dots or stripes of three-color phosphor layers that emit blue, green, and red light components is formed on the inner surface of the panel. A rectangular shadow mask is arranged on the inner side of the phosphor screen to oppose it. The shadow mask has a rectangular mask body and a mask frame attached to the skirt portion of the mask body. The mask body has a rectangular effective area which is formed spherically to have a predetermined radius of curvature and in which a large number of electron beam apertures are formed with a predetermined arrangement, and a non-effective area which is located around the effective area and in which no electron beam apertures are formed. The skirt portion is formed continuously to the non-effective area. The shadow mask is supported on the inner side of the panel such that its effective area opposes the phosphor screen, by engaging a frame holder, attached to the mask frame, with stud pins provided on the panel. An electron gun for emitting three electron beams toward the phosphor screen is disposed in the neck of the funnel.
The shadow mask serves to select three electron beams emitted from the electron gun so that they are correctly land on the corresponding ones of the three-color phosphor layers. In the cathode-ray tube, the three electron beams emitted from the electron gun are deflected by a magnetic field generated by a deflecting yoke which is mounted on the outer surface of the funnel, and the deflected electron beams are selected by the shadow mask to scan the phosphor screen in the horizontal and vertical directions, thereby displaying a color image.
In color cathode-ray tubes having this arrangement, one having a flat screen is recently put into practical use. In the color cathode-ray tube of this type, as the screen surface is made flat, the effective area of the shadow mask is also made flat. Therefore, doming of the effective area toward the phosphor screen, which occurs when the shadow mask is subjected to thermal expansion by impingement of the electron beams against the shadow mask, becomes large. In this case, landing errors among the three electron beams on the three-color phosphor layers are large, leading to a degradation in image quality.